1. Field of the Invention
This invention relates to cathode ray tubes (CRTs) and more specifically to a CRT in which a pair of electron guns are used to write and erase an image on a target via secondary electron emission.
2. Description of the Related Art
Electron guns are used to write and erase a charge pattern onto a beam-addressing surface of a light valve target. The charge pattern imparts a modulation onto a light beam in proportion to the pixel intensities and directs the modulated light beam through projection optics to form a video display. Such beam-addressed light valve targets have been demonstrated using transmissive and reflective liquid crystals, reflective membranes, deformable mirror layers and pixelated micromirror arrays.
Most of these targets utilize the secondary electron emission characteristics of the addressing surface to write and erase the charge pattern. The addressing surface is characterized by a secondary electron emission curve that plots the emission coefficients xcex4 i.e. the ratio of emitted secondaries to incident primaries, against the landing energy of the primary electrons. At landing energies between first and second crossover points (xcex41), the surface exhibits a coefficient greater than one. Outside that region, the surface exhibits a coefficient less than one. In general, clean conductors have coefficients less than one and insulators have coefficients greater than one for useful beam energies.
In known systems, the write gun emits primary electrons that strike the target""s addressing surface with a landing energy above the first crossover causing more secondary electrons to be ejected than incident primary electrons. The secondaries are collected by a collector electrode (grid or plate), that is held at a relatively positive potential with respect to the addressing surface. This produces a charge pattern that has a positive net charge, which increases the pixel potentials and in turn actuates the liquid crystal, membrane, reflective layer or micromirror to modulate the light. The degree of modulation is controlled by changing the beam current.
In video applications, each charge pattern or frame must be erased prior to the next pass of the write gun. It is well known that the brightness of the light modulator is closely tied to the optical throughput of the target. In large part, optical throughput is determined by the frame time utilization of each pixel, i.e. how long the pixel is held in its modulated position before it is erased. Ideally, each pixel would be held at its intended modulated position until that pixel was to be rewritten and then instantaneously erased. This would maximize the amount of light passed through the projection optics while maintaining video performance.
A common erasure technique is RC decay, in which the deposited charge is bled off over the frame time. The device""s RC time constant must be short enough that the pixel intensity is erased prior to writing the next value in order to maintain video performance. The main drawback, however, is the fact that approximately two-thirds of the available light is lost due to RC decay. This greatly limits the display""s brightness and contrast capabilities.
In the 1950s, U.S. Pat. No. 2,682,010 to Orthuber and entitled xe2x80x9cCathode-Ray Projection Tubexe2x80x9d introduced scanning an electron beam over a transparent dielectric element suspended above an array of reflective xe2x80x9cflapsxe2x80x9d. The deposited charge pattern exerts electrostatic forces on the flaps causing them to deflect and form a projected image.
Orthuber suggests two possible ways to erase the charge pattern, the traditional RC decay as shown in his FIG. 3 and the use of a separate erase gun as shown in his FIG. 4. The erase gun operates between the first and second crossovers and leads the write beam by a short interval, such as one or two periods of the horizontal sweep frequency, so that each pixel is restored to reference potential shortly before being subjected to the write beam.
As shown in his FIG. 4, Orthuber suggests placing xe2x80x9ctwo complete beam generating or deflecting systemsxe2x80x9d, i.e. the write and erase guns in a single off-axis neck. Orthuber""s double bi-potential guns each have an emitter (cathode); a wehnelt suppressor electrode (biasing electrode), a focusing electrode, a set of vertical electro-static deflection plates and a set of horizontal electro-static deflection plates. Based upon the drawing, the Orthuber guns do not have standard triodes. Ordinarily, the triode is comprised of an emitter, a Wehnelt suppressor electrode and a first accelerator.
The Orthuber guns use the focusing electrode to function as both the focusing electrode and the first accelerator. This is extremely bad practice. In fact, this gun would not function properly. Since Orthuber shows a target that is tilted by 45xc2x0 the device would require dynamic focus voltage correction. If a dynamic voltage were applied to the focusing electrode, which functions as a first accelerator, then the cathode emission would modulate uncontrollably.
Orthuber also shows horizontal deflection plates directly in front of the focusing electrode. The volume between the focusing electrode and the horizontal deflection plates forms the main lens of a bi-potential type electron gun. The main lens in Orthuber""s gun is not rotationally symmetric. This configuration would cause uncontrollable astigmatism and cause the electron gun to not function properly.
By placing two complete beam generating systems in a single neck in the manner depicted in FIG. 4, Orthuber""s CRT would require a specially designed and manufacturer stem to bring the external potentials inside the single neck. Orthuber""s design would require a 22-pin rotation stem with two isolated high voltage pins (one for each focusing potential), four open pins (two on each side of the high voltage pins) and sixteen low voltage pins. This would be very difficult to fit inside the neck glass without having arcing between the pins and would be very expensive to manufacture.
In the early 1970s, Westinghouse Electric Corporation developed an electron gun addressed cantilever beam deformable mirror device, which is described in R. Thomas et al., xe2x80x9cThe Mirror-Matrix Tube: A Novel Light Valve for Projection Displays,xe2x80x9d ED-22 IEEE Tran. Elec. Dev. 765 (1975) and U.S. Pat. Nos. 3,746,310, 3,886,310 and 3,896,338. A low energy scanning electron beam deposits a charge pattern directly onto cloverleaf shaped mirrors causing them to be deformed toward a reference grid electrode on the substrate by electrostatic actuation. Erasure is achieved by raising the target voltage to equal the field mesh potential while flooding the tube with low energy electrons to simultaneously erase all of the mirrors, i.e. the whole frame. This approach improves the modulator""s FTU but produces xe2x80x9cflickerxe2x80x9d, which is unacceptable in video applications.
At the same time IBM was developing the DSDT as described by James Ross and Eugene Kozol""s paper entitled xe2x80x9cPerformance Characteristics of the Deformographic Storage Display Tube (DSDT)xe2x80x9d IEEE Intercon Technical Papers, Session 7, pp. 1-8, 1973. The DSDT is a dielectric membrane (target), which consists of an electronically controllable storage substrate, a deformable material layer, and a reflective layer. The target is mounted in the tube envelope so the storage substrate faces the electron gun. The deformable material with its conformal reflective layer is isolated in the separate front chamber of the tube. Deformations are created in the deformable material as the result of negative electrostatic charges deposited by the on-axis write gun, which is operated above the second crossover. These deformation are converted into a visual image by the off-axis schlieren optical systems. The charge pattern is erased by an off-axis flood erase gun that operates between the first and second crossovers. Electronic control of these guns provides for storage mode, variable persistence mode and selective erase modes of control.
In the late seventies and early eighties, Tektronix pioneered the development of an electron beam addressed liquid crystal light valve of the cathode-ray tube type as described in Duane A. Haven, IEEE Transactions on Electron Devices, Vol. ED-30, No. 5, 489-492, May 1983. Haven""s light valve is a form of cathode-ray tube (CRT) having a twisted nematic liquid crystal cell, one substrate surface of which serving as a target for a writing electron beam propagating in the tube. The target substrate comprises a thin sheet of dielectric material and forms one face of the liquid crystal cell.
The CRT also includes a writing electron gun, a flood electron gun, and a ring-type collector electrode positioned adjacent the periphery of the target surface. The flood electron gun maintains the target surface of the cell at a desired operating electrostatic potential VFG, which is the potential of the flood electron gun cathode. Polarized light propagating from an external source enters the CRT through an optically transparent entry window on one side of the tube and passes through the cell and out through an exit window. The writing and flood guns are mounted at oblique angles relative to the target substrate to keep them out of the light path. Unwritten areas of the liquid crystal cell remain in an xe2x80x9cOFFxe2x80x9d state that rotates by 90 degrees the polarized direction of the light emanating from the external source. Areas addressed by the writing beam are temporarily switched into an xe2x80x9cONxe2x80x9d state that leaves unchanged the polarization direction of the light emanating from the external source. This creates a light image pattern that is detected by an analyzing polarizer positioned in the path of light exiting the exit window.
The transparent collector electrode of the light valve of Haven is operated at a potential VCOL, which is positive relative to the potential VFG of the target surface. The flood gun electrons strike the target surface with an energy that is below the first crossover point on the secondary electron emission ratio curve for the dielectric material forming the target surface. Under these conditions, the electrostatic potential of the target surface is stabilized to the potential of the flood gun cathode. The writing gun is operated under conditions so that the writing beam electrons strike the target surface with an energy that is above the first crossover point but below the second crossover point of the dielectric material.
When the writing beam strikes the target surface, secondary emission causes the written area to charge positive relative to the unwritten areas of the target surface, which are at the flood gun potential VFG. The potential of the written area rises, approaching the potential VCOL of the collector electrode and driving the liquid crystal cell into the xe2x80x9cONxe2x80x9d state. After the writing beam is turned off, the potential drops back to the flood gun cathode potential VFG and allows the liquid crystal cell to relax to the xe2x80x9cOFFxe2x80x9d state. This occurs because VCOL is below the first crossover point and more electrons are absorbed than are emitted from the previously written area.
The ring-type collector electrode is positioned adjacent the periphery of the liquid crystal cell and outside the projection light path through the valve. There is a relatively large separation between the collector electrode and the central areas of the target surface, Which separation causes the collection of secondary electrons emitted from the central areas on the target surface to be relatively inefficient. The reason for such inefficiency is that secondary electrons emitted from the central areas on the target surface redeposit on the positively charged, previously written areas of the target surface. This redeposition of secondary electrons at least partly erases the written image, thereby reducing the resolution and contrast capability of the light valve.
A different technique for erasing a beam addressed liquid crystal light valve is presented in U.S. Pat. Nos. 5,765,717 and 5,884,874. The transparent collector electrode is segmented into four or more electrically isolated segments. As the erase and write guns raster scan the light valve, with the write gun lagging by two segments, a controller switches the potentials on the segments above the erase and write guns to ground and to +300V with respect to the incident surface. Since both guns operate at energies above the crossover point, the erase gun secondaries will redeposit themselves over the segment thereby erasing the charge pattern and the write gun secondaries will be collected by the segment thereby writing a new charge pattern.
Although this approach provides improved resolution and contrast, it requires a segmented grid and a synchronized controller. Since both guns operate above the cross-over, image resolution can be further improved by coating the entire surface of the LCLV with a material such as magnesium oxide (MgO), which exhibits a very high emission coefficient, as described in U.S. Pat. No. 4,744,636. However, the best FTU that can be achieved using the segmented grid is (nxe2x88x922)/n where n is the number of segments. For example, a 4 segment grid would have only a 50% FTU.
In view of the above problems, the present invention provides a dual-gun, single neck CRT that focuses two beams, with significantly differing energies, onto a secondary emission target while maintaining compatibility with a standard fourteen-rotation stem and achieving FTU in excess of 95%.
This is accomplished by sharing certain gun parts while maintaining independent control over others. A pair of Einzel guns (write and erase) are mounted in parallel and aligned in the vertical direction rather than the horizontal inside the CRT. The write and erase guns are configured to share a common second accelerator electrode, a common final accelerator electrode, mounting beads and a magnetic deflection yoke. Inside each gun, a single stem pin is connected to the negative filament lead and the cathode. The bias on the erase gun""s positive filament is stepped up to lower the effective landing energy of the erase beam. The write and erase guns have different sized main lenses and sit off-axis by differing amounts. To compensate, the focus voltages are independently adjusted so that both the write and erase beams have the same focal length.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: